Personally I would like to see the following:
A break down of the astartes implants and an actual game function asigned to each one. A bit like the techpriest trait. More than simply 'unnatural strength and toughness'. Back when I playtested DH I created a list of the different organs and assigned them an ability on the assumption that a character started from a scout and moved up.
This is I feel an important aspect the RPG should address. Those implants are more than just a bonus to a stat, they are the reason a marine exists. They hold a holy reverance for them.
Reflection of indoctrination beyond just 'Mental Fortress' or 'Iron Discipline' talents. A marine's psychology is very specialised and also specific to their chapter. These I feel can be reflected in the rules of each character.
The other is individualisation of power armour. I would like to see power armour broken down into location with different marks offering different modifiers and upgrades. Ie a marine might be wearing Mk IV greaves, Mk VI pauldrons and Mk VII torso, arms and helmet.
Power armour is like a spiritual partner for a space marine, their other half. If a marine could be married to anything it would be their armour. They have an intimate knowledge of one another that produces a warrior greater than the sum of its parts. That reverence should be reflected in more detailed armour rules I feel.
Afterall, marines have a much narrow range of options than a normal human warrior so extra focus on specific important wargear items could go a long way to fill the deficit.
I can't imagine they won't have chapter traits as part of starting creation. It's an obvious choice. But the above are my biggest wants because to me those represent a space marine more completely than anything else. Not the gun, or the chainsword, but the implants armour and indoctrination.
So, what would you like to see in the Deathwatch book?
Yeah, right. So a "military power source" is apparently not as limited but unfortunately it is not listed or explained anywhere.